Staphylococcus aureus and S. epidermidis are important human pathogens, accounting for the majority of nosocomial sepsis cases reported each year. Both are associated with colonization of implanted medical devices, leading to systemic bacteremia and potentially fatal conditions such as endocarditis and ventilator-associated pneumonia. Due to the increase in the number of organisms exhibiting antibiotic resistance, particularly methicillin and vancomycin resistant S. aureus strains, new avenues of protection are sought. Of particular appeal are bacterial vaccines that target surface molecules known to be important in mediating initial steps of adherence and accumulation.
The primary stages of attachment and accumulation have mostly been studied in the coagulase negative staphylococci (CoNS) such as S. epidermidis. While initial attachment occurs mainly through non-specific interaction with bacterial surface proteins, accumulation is predominantly mediated by bacterial exo-polysaccharides. These surface-expressed carbohydrates may be attractive targets for vaccine-based strategies.
One staphylococcal carbohydrate of interest is the polysaccharide intercellular adhesin (PIA). PIA has been shown to be a major constituent of biofilm, an amorphous exopolysaccharide produced during the accumulation phase of growth. It is responsible for mediating cell-cell adhesion and may function to shield the growing colony from host immune defenses.
PIA isolated from S. epidermidis has been characterized by Mack et al, 1996 J. Bact. 178(1): 175–183. It is a polymer of molecular mass less than 30,000 Da, composed of β-1,6-linked glucosamine. Approximately 85% of the amino groups are substituted with N-acetate, and a minor proportion of the sugar hydroxyl groups are esterified with phosphate and succinate.
PIA production is associated with the ica gene locus and has been demonstrated in vitro under certain defined growth conditions for several CoNS strains. Recent reports have shown that other staphylococcus species, including S. aureus, contain the ica locus. Some of these were able to elicit biofilms in culture while others did not. All of the environmental factors affecting regulation of ica are not known, and a major impediment to the development of polysaccharide-based vaccines has been the sensitivity of their expression to growth conditions. Hence, it has been difficult to show with certainty that organisms grown in vitro produce the same surface polysaccharides as those present in an in vivo infection. Finally, accurate structural characterization of polysaccharide preparations has often been hampered by residual media contaminants, and this has occasionally led to identification of what were believed to be novel antigens.
It would therefore be desirable to have a simplified, robust process for the production of SAE in high amounts to facilitate vaccine production. Furthermore, it would be desirable to have chemical characterization of isolated SAE from sources other than S. epidermidis, namely, from S. aureus. 